Off-line switching power supplies convert an alternating current (AC) supply voltage to a direct current (DC) output voltage. The power factor for such an off-line switching power supply is defined as the ratio of the actual input power (actual input voltage times actual input current) to the root-mean-square (RMS) input power (RMS input voltage times RMS input current). Although ideally the power factor for such a supply should be equal to one, in practice, the power factor is generally much less than one.
This decrease in power factor occurs because, although the actual input voltage varies sinusoidally, the actual input current does not. The actual input current is distorted relative to the actual input voltage. This distortion of the current from a sinusoidal waveform is due to the fact that the actual input current is drawn by the rectifier and the filter of the power supply as a series of current pulses which occur at the peak of the input voltage waveform. These current pulses cause the actual input power to be less than the RMS input power, and result in power factor values for such systems typically to range from 0.5 to 0.7.
Various methods have been developed to restore the power factor near unity. Referring to FIG. 1, one method known to the prior art utilizes a boost converter 30 controlled by a pulse width modulator (PWM) 40. Although the boost converter 30 is shown having an inductor 60 and a diode 62, other forms of boost conversion are contemplated. An input voltage sensing circuit 80 measures the input rectified AC line voltage (V.sub.in) 50 and derives an signal (V.sub.AC) 54 from it.
FIG. 1A depicts an embodiment of an input voltage sensing circuit 80 utilizing two resistors as a voltage divider to provide a voltage which varies as the line voltage. It is also possible to utilize a single resistor to produce a current which varies as the line voltage. Still other embodiments are possible.
Similarly, an output voltage sensing circuit 90 measures the output voltage (V.sub.o) 56 to produce a signal (V.sub.os) 58. FIG. 1B depicts an embodiment of an output voltage sensing circuit 90 utilizing two resistors as a voltage divider. Still other embodiments are possible. The signal (V.sub.os) 58 is compared to a reference voltage (V.sub.ref) 100 by an amplifier 110 and, an error signal (V.sub.e) 112 is generated. This error signal (V.sub.e) 112 is multiplied by the signal (V.sub.AC) 54 of input voltage sensing circuit 80 to generate a multiplier signal (V.sub.m) 114. A amplifier 130 compares this multiplier signal (V.sub.m) 114 to a signal (V.sub.i) derived from the input current (I.sub.in) 118, as measured by a current sensor 140, and generates a control signal (V.sub.c) 142.
FIG. 1C depicts an embodiment of a current sensor 140 which uses a resistor to produce the voltage drop (V.sub.i) which is proportional to the current (I.sub.in). Still other embodiments, such as those using a transformer, are also possible.
This control signal (V.sub.c) 142 governs the operation of the PWM 40 in a manner well known to one skilled in the art, and hence controls the switching of the boost converter 30 through switch 144. This control loop is such that the instantaneous input current (I.sub.in) 118 is forced to follow the multiplier signal (V.sub.m) 114 which contains both the waveform and amplitude information necessary for good power factor control and voltage regulation.
The multiplier signal (V.sub.m) 114 is governed by the equation: EQU V.sub.m =k.sub.2 * V.sub.e * V.sub.AC,
where k.sub.2 is a constant which includes all the linear gain terms in the control loop.
Since the input current (I.sub.in) 118 is forced to follow the multiplier signal (V.sub.m) 114, the input current (I.sub.in) 118 is governed by the equation: EQU I.sub.in =k.sub.1 * V.sub.m
where k.sub.1 is a constant which includes all the terms from the boost converter 30, the pwm 40 and the amplifier 130.
Substituting the defining equation for V.sub.m into this equation yields the relationship: EQU I.sub.in =k.sub.1 * k.sub.2 * V.sub.e * V.sub.AC.
This result means that the circuit described will meet the basic requirement of generating a sinusoidal input current (determined by the term V.sub.AC) and a constant output voltage (determined by the term V.sub.e).
However, the overall performance of this circuit will be less than optimum because, when the circuit is operated under the conditions of a constant load, the circuit should deliver constant power, regardless of changes the input voltage. Further, since there is minimal power lost within the boost converter itself, the average input power should also remain relatively constant.
The constraint of delivering constant power when under constant load, for any value of RMS input line voltage, means that the circuit should compensate for an increase in the RMS input line voltage with a corresponding reduction of RMS input current. However, since, as shown above, the input current is governed by expression: EQU I.sub.in =k.sub.1 * k.sub.2 * V.sub.e * V.sub.AC
an increase in V.sub.AC 54, which is determined by the input RMS line voltage, will cause an increase in input current (I.sub.in) 118, rather than a decrease, for constant values of k.sub.1 and k.sub.2. Additionally, with this increase in current, the average input power does not remain constant, but will increase as the square of the input voltage (V.sub.in), unless it is corrected by a compensating change in the error term (V.sub.e).
With sufficient gain in the voltage feedback loop, the error voltage (V.sub.e) can be forced to correct for this change in line voltage (V.sub.AC) although only at significant cost in dynamic performance. Additionally, the use of gain for line voltage correction causes both the maximum output power limit and the AC gain of the voltage feedback loop to vary as V.sub.in.sup.2, making the trade-off between good dynamic performance and low input current distortion impossible to achieve over a wide range of line voltages.
The ability to use the power supply over a wide range of line voltages, is an important consideration in the design of power supplies since the variation in RMS line voltages throughout the world is, for example, from about one hundred volts in Japan to about 240 volts in Great Britain. Thus the ability of a circuit to achieve high power factor in the presence of such a range of input line voltages would permit a single off-line power supply design to be used in all locations throughout the world.
Thus it would be desirable to provide a circuit which achieves high power factor over a wide range of input line voltages.